Erbium Doped Fibre Amplifiers (EDFAs) are widely deployed in optical Dense Wavelength Division Multiplexing (DWDM) transmission systems due to their capability to amplify all DWDM signal simultaneously in an almost satisfying manner. Furthermore, EDFAs represent key components for the integration of point-to-point links in optical networks. This trend results in an increased number of amplifiers used in an optical network due to the additional losses introduced by switches, multiplexers etc. Furthermore, optical networking technology has become attractive for regional and metropolitan networks with stringent cost requirements.
Therefore, reducing amplifier cost without sacrificing functionality is a key requirement for EDFAs. The core of the amplifier is given by the gain medium, i.e. the “Erbium Doped Fibre”, that provides the amplification, and by the laser pumps, that provide the power to allow that a sufficient part of the dopant ions in the Erbium Doped Fibre (EDF) leave the ground state to reach the desired excited state level. Practical implementations of EDFAs deploy additional passive components such as tap couplers/splitters, signal monitors, WDM couplers to couple the pump radiation and the signal light into the EDF, isolators, etc, too. However, from a costs perspective, pump lasers represent the driving element. As a consequence, the number of pumps must be kept as low as possible to significantly reduce EDFA costs.
In order to reduce cost of an EDFA, it has been proposed that two or more EDF coils are pumped by the same laser, making use of a fused splitter characterized by a fixed splitting ratio so that the portions of the light entering each EDFA stage cannot be set independently.
On the other hand, a four port combiner with unequal branching ratio has been used in a configuration with two pumps in order to compensate for deviations of component characteristics from design values and to improve reliability. Embodiments of this invention are described in U.S. Pat. No. 5,561,552 and EP 588 557 A1.
However, there is a major drawback associated with this technique. In most cases, WDM systems are put into operation with a small number of active channels. Later on, additional channels are added according to the demand of capacity. Therefore, amplifiers and their control algorithm have to be designed to provide suitable performance starting from operation with a single channel to up to the maximum channel load.
The noise figure of setups based on pump slitting that are operated at constant gain is significantly larger at small input power, i.e. small channel count. Thus, the maximum number of spans is limited by the noise performance at small channel count. To reduce this disadvantage, high values of the splitting ratio can be considered. However, to achieve high output power levels (that is the case for high channel load) lower values of the splitting ratio are required (i.e. high pump power in the second EDF coil). As a consequence, high values of the splitting ratio are related to lower output power levels, i.e. the maximum gain provided by the amplifier is reduced. The choice of a fixed splitting ratio is based on a trade-off between these two constraints.
In order to reduce the limitations imposed by a fixed splitting ratio, a configuration is described in a patent application DE 10 2005 031 897 A1. For this setup, the pump power entering the second EDF coil is not a fixed portion of the power provided by the pump, because the attenuation provided by an EDF (acting as a saturated absorber) in the pump line of the second stage decreases by increasing the pump power. However, this solution presents the following drawbacks:    The EDF in the pump path causes absorption of the pump light.    The EDF in the pump path acts as an Amplified Spontaneous Emission (ASE) noise source that might be added (after attenuation by the WDM coupler) to the signal propagating in the optical main path. In addition, backward propagating ASE will propagate in the direction of the pump.    Once the fixed splitting ratio and the length of the EDF in the pump path have been fixed, the pump power impinging the second EDF stage is a function of the power provided by the pump itself. As a consequence, the equivalent splitting ratio of this setup is a function of the pump power, i.e. it is not possible to set the equivalent splitting ratio and the power provided by the pump independently. In other words, this architecture has one degree of freedom. But this sets a limit to the maximum value for the equivalent splitting ratio that is responsible for a suboptimal noise performance.